1. Field of the Invention
The present invention relates generally to processes for recovering precious metals from ore containing the same and, more particularly, to processes for recovering precious metals such as silver and gold from low grade ore. Specifically, the present invention relates to an improved process for the extraction of precious metals by lixiviation from ores containing the same, which process is highly efficient and environmentally safe.
2. Description of the Prior Art
There are many processes and techniques for recovering precious metals such as silver, gold, platinum and the like, from ores containing these metals. The particular technique used will frequently depend upon the type of ore containing the precious metal as well as the grade of ore, i.e., the higher the grade of ore, the greater the concentration of precious metal therein. Low grade ore creates more problems during processing in that the net yield of precious metal per unit weight of ore is less. Moreover, there is more non-precious metal material present in low grade ore which may interfere with various processing stages.
Lixiviation is a technique widely used to extract a soluble component from a solid mixture by the leaching of that component from the solid. Lixiviation utilizing various cyanide solutions is an extremely popular technique for extracting precious metals from their ores. However, such cyanide solutions are highly toxic thereby requiring special handling procedures. They are also highly damaging to the environment. Moreover, cyanides are costly and are difficult to use with certain types of ores, especially those ores containing copper and/or manganese, since these materials readily contaminate the cyanide. Since such contamination materials are frequently present in varying degrees in most ores, there is a substantial loss rate with the cyanide solutions as well as relatively poor recovery rates from the precious metals. As a result, there has been considerable effort expended in attempting to find alternate lixiviation processes which do not utilize cyanide based solutions.
There are many difficult-to-treat ores in existence which contain manganese, copper oxides, and various other constituents along with significant quantities of silver, gold and other precious metals. It would be highly desirable to be able to extract these precious metals from these ores if a suitable and sufficiently inexpensive technique existed for such recovery. However, the present techniques, and in particular cyanide-based techniques, are simply not adequate, and these ores remain for the most part an untapped mineral resource.
Various lixiviating agents other than cyanide have been looked at as potential replacements for cyanide-based solutions. One such process is disclosed in U.S. Pat. No. 4,369,061, issued to KERLEY, JR. on Jan. 18, 1983, which process utilizes ammonium thiosulfate liquor as the lixiviating agent in the presence of copper. Other processes, including that disclosed in U.S. Pat. No. 4,342,591, issued to LESOILLE on Aug. 3, 1982, utilize thiourea as a lixiviating agent. It has been found that thiourea extracts precious metals from ore at a substantially faster rate than do cyanide solutions. However, certain disadvantages to thiourea lixiviating solutions are that they tend to cost substantially more than cyanide-based solution systems, and they also tend to be more selective than cyanide. Thus, existing thiourea lixiviants are generally not appropriate for use with a number of different types of ores, for instance carbonate ores. Consequently, there still exists a significant need for an efficient, cost effective, and environmentally safe extraction technique for precious metals to replace the state of the art techniques utilizing cyanide solutions and the like.